Side weir is a common hydraulic structure installed along the main channel to divert a part of the flow from the main channel. While the depth of water reaches the side weir crest’s elevation, the weir starts to overflow thereby evacuates a portion of main channel flow discharge to the lateral channel. Side weirs are used in irrigation and drainage channels, flood control systems and wastewater collection networks to divert the flow as well as to isolate the sedimentation and to reduce the bed loads. Labyrinth side weirs are a type of lateral weirs with a discharge coefficient of 1.5–4.5 times the rectangular side weirs. During the last decade, these weirs have been the interest of different researches. In this study, labyrinth side weirs besides groups of vane plates were modeled numerically using an open source software namely OpenFOAM-2.1. The free-surface flow over the triangular labyrinth side weirs was modeled using Volume of Fluids (VOF) method. To calculate the flow characteristics of subcritical approach flow condition the k- e standard turbulence model was applied. The predictions of 3D numerical turbulence model were validated using the experimental results of former investigations. The objective of the present study was to improve the flow field around the labyrinth side weirs by employing a group of vane plates with different arrangements.The pressure gradient around the plates creates secondary flow regions up and downstream of the vane plates resulting in a higher flow discharge toward the lateral channel. Furthermore, these plates are influential tools to control the sedimentation. Results indicate that the maximum discharge coefficient is responsible while the plates are located perpendicular to the flow direction at the downstream end of the weirs inside the main channel. It was found that to have a better performance of plates the ratio of the plates width to the channel width should be almost 0.38-0.4. Due to reduction of momentum, the greatest region of separation was observed around the furthest plate. Stagnation regions occur at the downstream end of the weir inside the channel. Finally, for the best arrangement of plates, free-surface profiles, velocity distribution, flow streamlines, vortices, stagnation regions, the angle of spilling jet, the Reynolds stresses and the turbulent kinetic energy were analyzed in details. Key words: Discharge coefficient, flow field, Labyrinth side weir, OpenFOAM, Vane plates.